How A Polarimeter Works: The Simple Explanation

Imagine tying a piece of thick rope to a hook in a wall, and then shaking the rope vigorously. The rope will be vibrating in all possible directions - up-and-down, side-to-side, and all the directions in-between - giving it a really complex overall motion. Now, suppose you passed the rope through a vertical rectangular hole, like this: []. The rope has a really tight fit in the hole. The only vibrations still happening at the other side of the hole will be vertical ones. All the others will have been prevented by the hole.

What emerges from the hole could be described as "plane polarized rope", because the vibrations are only in a single (vertical) plane. Now look at the possibility of putting a second hole on the rope. If it is aligned the same way as the first one, the vibrations will still get through. But if the second hole is at 90° to the first one (so horizontally), the rope will stop vibrating entirely to the right of the second hole. The second hole will only let through horizontal vibrations - and there aren't any.

Light is also made up of vibrations - this time, electromagnetic ones. Some materials have the ability to screen out all the vibrations apart from those in one plane and so produce plane polarized light. The most familiar example of this is the material that Polaroid sunglasses are made of. If you wear one pair of Polaroid sunglasses and hold another pair up in front of them so that the glasses are held vertically rather than horizontally, you'll find that no light gets through - you will just see darkness. This is equivalent to the two holes at right angles in the rope analogy. The polaroids are "crossed". (This not exactly the way Polaroid glasses work, but it gives a good idea).

A polarimeter works the same way: You have two polaroid glasses, like the two holes with the rope, one glass is the polarizer, the other glass is the analyzer. The polarizer ensures that only a beam of polarized monochromatic light (light of only a single frequency - in other words a single color) is passed through the solution behind the polarizer in the polarimeter. After the tube with the solution is the analyzer. The polarimeter is originally set up with water in the tube. Water isn't optically active - it has no effect on the plane of polarization. The analyzer is rotated until you can't see any light coming through the polarimeter. The polaroids are then "crossed".

An optically active substance is a substance which can rotate the plane of polarization of plane polarized light. If you shine the polarized monochromatic light through a solution with an optically active substance, then light emerges: its plane of polarization is found to have rotated. The substance rotates the plane of polarization of the light, and so the analyzer won't be at right-angles to it any longer and some light will get through. You would have to rotate the analyzer in order to cut the light off again.

The rotation may be either clockwise or anti-clockwise. Assuming the original plane of polarization was vertical, you can easily tell whether the plane of polarization has been rotated clockwise or anti-clockwise, and by how much.


Author Name: Kathy Brasch : Nationalmicroscope.com

How A Polarimeter Works: The Simple Explanation